Point mutations at N434 in D1-S6 of mu1 Na(+) channels modulate binding affinity and stereoselectivity of local anesthetic enantiomers

[Stocktaking of local anesthetics 2020]

For decades local anesthetics have proven to be safe and effective drugs in the clinical practice, crucially promoting the enormous achievements in regional anesthesia. Meanwhile, it is a well-known fact that local anesthetics are much more than just "simple" sodium channel blockers. They also interact with numerous other ion channels and subcellular structures, enhancing nerve blockade and resulting in systemic "alternative" effects, which can sometimes even be clinically used. By the simultaneous administration of various adjuvants (e.g., opioids, corticosteroids and α₂-receptor agonists) attempts are made to prolong the time of action of local anesthetics after a single administration in order to achieve the best possible improvement in postoperative analgesia. In this context, ultralong-acting local anesthetics, such as liposomal bupivacaine, which at least theoretically can provide a sensory nerve block for several days, have been developed and clinically introduced. The coming years will show whether these approaches will develop into genuine alternatives to the personnel and cost-intensive continuous nerve blockades.Local anesthetic-induced systemic toxicity is meanwhile rare but still a potentially life-threatening event, frequently resulting from accidental intravascular injection or extensive systemic resorption. Consequently, slow and fractional application of these agents with intermittent aspiration helps to prevent toxic sequelae. If toxic symptoms occur, however, the intravenous infusion of 20% lipid solutions in addition to basic treatment measures can enhance the success of treatment.

Mexiletine Block of Voltage-Gated Sodium Channels: Isoform- and State-Dependent Drug-Pore Interactions

Mexiletine is a class Ib antiarrhythmic drug and is also used clinically to reduce or prevent myotonia. In addition, mexiletine has neuroprotective effects in models of brain ischemia. We compared state-dependent affinities of mexiletine for Na_v1.2, Na_v1.4, and Na_v1.5 and examined the effects of mutations of transmembrane segment S6 residues on mexiletine block of Na_v1.5. Three channel isoforms had similar affinities of mexiletine for the rested state, and Na_v1.4 and Na_v1.5 had similar affinities for the open and inactivated states, while Na_v1.2 had lower affinity for these states than Na_v1.4 and Na_v1.5. Mutational studies revealed that the largest affinity change was observed for an Ala substitution of Phe in domain IV S6. In our homology modeling based on the bacterial Na⁺ channel, mexiletine changed its location and orientation in the pore depending on the state of the channel, irrespective of the channel isoform. Mexiletine occurred in the upper part in the pore in the open state and lower in the closed state. High-affinity binding of mexiletine in the open states of Na_v1.4 and Na_v1.5 was caused by a π-π interaction with Phe, whereas mexiletine was located away from Phe in the open state of Na_v1.2. These results provide crucial information on the mechanism of isoform differences in state-dependent block by local anesthetics and related drugs. Mexiletine at upper locations in the open state may effectively cause an electrostatic mechanism of block.

Mutational analysis of state-dependent contacts in the pore module of eukaryotic sodium channels

Voltage-gated sodium channels have residues that change or may change contacts upon gating. Contributions of individual contacts in stability of different states are incompletely understood. Pore-lining inner helices contain exceptionally conserved asparagines in positions i20. Here we explored how mutations in positions i20 and i29 affect electrophysiological properties of insect sodium channels. In repeat interfaces I/IV, III/II and IV/III, alanine substitutions caused positive activation shifts in positions i20 and i29, negative shifts of slow inactivation in positions i20 and positive shifts of slow inactivation in positions i29. The results support the hypothesis on open state inter-repeat H-bonding of residues i20 and i29. The shift magnitudes vary between interfaces, reflecting structural asymmetry of the channels. Mutations in positions i20 of repeats III and IV caused much longer recovery delay from the slow and fast inactivation than other mutations. In repeat IV, alanine substitution of tyrosine i30 rescued positive activation shift of mutation in position i29. Our data suggest that polar residues in positions i29 are involved in stabilization of both the open and slow-inactivated states. Transition between the states may involve switching of H-bonding partners of residues i29 from the conserved asparagines to currently unknown residues.

Inhibitory Effect of Amitriptyline on the Impulse Activity of Cold Thermoreceptor Terminals of Intact and Tear-Deficient Guinea Pig Corneas

PURPOSE

Chronic dryness of the ocular surface evokes sensitization of corneal cold-sensitive neurons through an increase of sodium currents and a decrease of potassium currents, leading to the unpleasant dryness and pain sensations typical of dry eye disease. Here, we explored the effects of amitriptyline, a voltage-gated Na⁺ channel blocker used for the treatment of depression and chronic pain, on nerve terminal impulse (NTI) activity of cold-sensitive nerve terminals recorded in intact and tear-deficient guinea pig corneas.

METHODS

Main lachrymal gland was surgically removed in anesthetized guinea pigs to induce chronic tear deficiency. Four to 6 weeks afterward, animals were sacrificed and both corneas placed in a perfusion chamber superfused at 34°C. Thermal stimuli were induced by changing the solution temperature from 34°C to 20°C (cooling ramp) and from 34°C to 50°C (heating ramp). Spontaneous and stimulus-evoked NTIs of cold-sensitive nerve terminals were recorded before, during, and after perfusion with solutions containing amitriptyline at different concentrations (3-30 μM).

RESULTS

Perfusion with amitriptyline inhibited irreversibly and in a concentration-dependent manner the spontaneous NTI activity of cold thermoreceptors of intact corneas. This effect was less evident in tear-deficient corneas. In addition, amitriptyline (10 μM) attenuated the maximal response to cooling ramps without changing cold threshold in intact but not in tear-deficient corneas. Only cold thermoreceptors with low cooling threshold values were sensitive to amitriptyline.

CONCLUSION

Amitriptyline effectively reduces the activity of cold thermoreceptors, although its efficacy is different in intact and tear-deficient corneas, which might be due to the changes induced by ocular dryness in the expression of the various voltage-gated Na⁺ channels responsible of the action potential generation and propagation.

Mutagenesis of the NaChBac sodium channel discloses a functional role for a conserved S6 asparagine

Asparagine is conserved in the S6 transmembrane segments of all voltage-gated sodium, calcium, and TRP channels identified to date. A broad spectrum of channelopathies including cardiac arrhythmias, epilepsy, muscle diseases, and pain disorders is associated with its mutation. To investigate its effects on sodium channel functional properties, we mutated the simple prokaryotic sodium channel NaChBac. Electrophysiological characterization of the N225D mutant reveals that this conservative substitution shifts the voltage-dependence of inactivation by 25 mV to more hyperpolarized potentials. The mutant also displays greater thermostability, as determined by synchrotron radiation circular dichroism spectroscopy studies of purified channels. Based on our analyses of high-resolution structures of NaChBac homologues, we suggest that the side-chain amine group of asparagine 225 forms one or more hydrogen bonds with different channel elements and that these interactions are important for normal channel function. The N225D mutation eliminates these hydrogen bonds and the structural consequences involve an enhanced channel inactivation.

Asymmetric synthesis of batrachotoxin: Enantiomeric toxins show functional divergence against NaV

The steroidal neurotoxin (-)-batrachotoxin functions as a potent agonist of voltage-gated sodium ion channels (Na_Vs). Here we report concise asymmetric syntheses of the natural (-) and non-natural (+) antipodes of batrachotoxin, as well both enantiomers of a C-20 benzoate-modified derivative. Electrophysiological characterization of these molecules against Na_V subtypes establishes the non-natural toxin enantiomer as a reversible antagonist of channel function, markedly different in activity from (-)-batrachotoxin. Protein mutagenesis experiments implicate a shared binding side for the enantiomers in the inner pore cavity of Na_V These findings motivate and enable subsequent studies aimed at revealing how small molecules that target the channel inner pore modulate Na_V dynamics.

Molecular Mechanism of Action and Selectivity of Sodium Ch annel Blocker Insecticides

Sodium channel blocker insecticides (SCBIs) are a relatively new class of insecticides that are represented by two commercially registered compounds, indoxacarb and metaflumizone. SCBIs, like pyrethroids and DDT, target voltage-gated sodium channels (VGSCs) to intoxicate insects. In contrast to pyrethroids, however, SCBIs inhibit VGSCs at a distinct receptor site that overlaps those of therapeutic inhibitors of sodium channels, such as local anesthetics, anticonvulsants and antiarrhythmics. This review will recount the development of the SCBI insecticide class from its roots as chitin synthesis inhibitors, discuss the symptoms of poisoning and evidence supporting inhibition of VGSCs as their mechanism of action, describe the current model for SCBI-induced inhibition of VGSCs, present a model for the receptor for SCBIs on VGSCs, and highlight differences between data collected from mammalian and insect experimental models.

Isolated pores dissected from human two-pore channel 2 are functional

Multi-domain voltage-gated ion channels appear to have evolved through sequential rounds of intragenic duplication from a primordial one-domain precursor. Whereas modularity within one-domain symmetrical channels is established, little is known about the roles of individual regions within more complex asymmetrical channels where the domains have undergone substantial divergence. Here we isolated and characterised both of the divergent pore regions from human TPC2, a two-domain channel that holds a key intermediate position in the evolution of voltage-gated ion channels. In HeLa cells, each pore localised to the ER and caused Ca²⁺ depletion, whereas an ER-targeted pore mutated at a residue that inactivates full-length TPC2 did not. Additionally, one of the pores expressed at high levels in E. coli. When purified, it formed a stable, folded tetramer. Liposomes reconstituted with the pore supported Ca²⁺ and Na⁺ uptake that was inhibited by known blockers of full-length channels. Computational modelling of the pore corroborated cationic permeability and drug interaction. Therefore, despite divergence, both pores are constitutively active in the absence of their partners and retain several properties of the wild-type pore. Such symmetrical ‘pore-only’ proteins derived from divergent channel domains may therefore provide tractable tools for probing the functional architecture of complex ion channels.

Mutations in the transmembrane helix S6 of domain IV confer cockroach sodium channel resistance to sodium channel blocker insecticides and local anesthetics

Indoxacarb and metaflumizone are two sodium channel blocker insecticides (SCBIs). They preferably bind to and trap sodium channels in the slow-inactivated non-conducting state, a mode of action similar to that of local anesthetics (LAs). Recently, two sodium channel mutations, F1845Y (F(4i15)Y) and V1848I (V(4i18)I), in the transmembrane segment 6 of domain IV (IVS6), were identified to be associated with indoxacarb resistance in Plutella xylostella. F(4i15) is known to be critical for the action of LAs on mammalian sodium channels. Previously, mutation F(4i15)A in a cockroach sodium channel, BgNav1-1a, has been shown to reduce the action of lidocaine, a LA, but not the action of SCBIs. In this study, we introduced mutations F(4i15)Y and V(4i18)A/I individually into the cockroach sodium channel, BgNav1-1a, and conducted functional analysis of the three mutants in Xenopus oocytes. We found that both the F(4i15)Y and V(4i18)I mutations reduced the inhibition of sodium current by indoxacarb, DCJW (an active metabolite of indoxacarb) and metaflumizone. F(4i15)Y and V(4i18)I mutations also reduced the use-dependent block of sodium current by lidocaine. In contrast, substitution V(4i18)A enhanced the action metaflumizone and lidocaine. These results show that both F(4i15)Y and V(4i18)I mutations may contribute to target-site resistance to SCBIs, and provide the first molecular evidence for common amino acid determinants on insect sodium channels involved in action of SCBIs and LA.

MTSET modification of D4S6 cysteines stabilize the fast inactivated state of Nav1.5 sodium channels

The transmembrane S6 segments of Na⁺ sodium channels form the cytoplasmic entrance of the channel and line the internal aspects of the aqueous pore. This region of the channel has been implicated in Na⁺ channel permeation, gating, and pharmacology. In this study we utilized cysteine substitutions and methanethiosulfonate reagent (MTSET) to investigate the role of the S6 segment of homologous domain 4 (D4S6) in the gating of the cardiac (Na_v1.5) channel. D4S6 cysteine mutants were heterologously expressed in tsA201 cells and currents recorded using whole-cell patch clamp. Internal MTSET reduced the peak Na⁺ currents, induced hyperpolarizing shifts in steady-state inactivation and slowed the recovery of mutant channels with cysteines inserted near the middle (F1760C, V1763C) and C-terminus (Y1767C) of the D4S6. These findings suggested a link between the MTSET inhibition and fast inactivation. This was confirmed by expressing the V1763C and Y1767C mutations in non-inactivating Na_v1.5 channels. Removing inactivation abolished the MTSET inhibition of the V1763C and Y1767C mutants. The data indicate that the MTSET-induced reduction in current primarily results from slower recovery from inactivation that produces hyperpolarizing shifts in fast inactivation and decreases the steady-state availability of the channels. This contrasted with a cysteine inserted near the C-terminus of the D4S6 (I1770C) where MTSET increased the persistent Na⁺ current at depolarized voltages consistent with impaired fast inactivation. Covalent modification of D4S6 cysteines with MTSET adduct appears to reduce the mobility of the D4S6 segment and stabilize the channels in the fast inactivated state. These findings indicate that residues located near the middle and C-terminus of the D4S6 play an important role in fast inactivation.

Voltage-dependent blockade by bupivacaine of cardiac sodium channels expressed in Xenopus oocytes

Bupivacaine ranks as the most potent and efficient drug among class I local anesthetics, but its high potential for toxic reactions severely limits its clinical use. Although bupivacaine-induced toxicity is mainly caused by substantial blockade of voltage-gated sodium channels (VGSCs), how these hydrophobic molecules interact with the receptor sites to which they bind remains unclear. Nav1.5 is the dominant isoform of VGSCs expressed in cardiac myocytes, and its dysfunction may be the cause of bupivacaine-triggered arrhythmia. Here, we investigated the effect of bupivacaine on Nav1.5 within the clinical concentration range. The electrophysiological measurements on Nav1.5 expressed in Xenopus oocytes showed that bupivacaine induced a voltage- and concentration-dependent blockade on the peak of I Na and the half-maximal inhibitory dose was 4.51 μmol/L. Consistent with other local anesthetics, bupivacaine also induced a use-dependent blockade on Nav1.5 currents. The underlying mechanisms of this blockade may contribute to the fact that bupivacaine not only dose-dependently affected the gating kinetics of Nav1.5 but also accelerated the development of its open-state slow inactivation. These results extend our knowledge of the action of bupivacaine on cardiac sodium channels, and therefore contribute to the safer and more efficient clinical use of bupivacaine.

A residue in the transmembrane segment 6 of domain I in insect and mammalian sodium channels regulate differential sensitivities to pyrethroid insecticides

Voltage-gated sodium channels are critical for electrical signaling in the nervous system. Pyrethroid insecticides exert their toxic action by modifying the gating of sodium channels. A valine to methionine mutation in the transmembrane segment 6 of domain I (IS6) of sodium channels from tobacco budworms (Heliothis virescens) has been shown to alter channel gating and reduce insect sodium channel sensitivity to pyrethroids. A valine to leucine substitution was subsequently reported in pyrethroid-resistant bedbug populations. Intriguingly, pyrethroid-resistant mammalian sodium channels possess an isoleucine at the corresponding position. To determine whether different substitutions at this position alter channel gating and confer pyrethroid resistance, we made valine to methionine, isoleucine or leucine substitutions at the corresponding position, V409, in a cockroach sodium channel and examined the gating properties and pyrethroid sensitivity of the three mutants in Xenopus oocytes. All three mutations reduced the channel sensitivity to three pyrethroids (permethrin, cismethrin and deltamethrin). V409M, but not V409I or V409L, caused 6-7mV depolarizing shifts in the voltage dependences of both activation and inactivation. V409M and V409L slowed channel activation kinetics and accelerated open-state deactivation kinetics, but V409I did not. Furthermore, the substitution of isoleucine with valine, but not with methionine nor leucine, at the corresponding position in a rat skeletal muscle sodium channel, rNav1.4, enhanced channel sensitivity to deltamethrin. Collectively, our study highlights an important role of residues at 409 in regulating not only sodium channel gating, but also the differential sensitivities of insect and mammalian sodium channels to pyrethroids.

Combined modifications of mexiletine pharmacophores for new lead blockers of Na(v)1.4 channels

Previously identified potent and/or use-dependent mexiletine (Mex) analogs were used as template for the rational design of new Na(v)-channel blockers. The effects of the novel analogs were tested on sodium currents of native myofibers. Data and molecular modeling show that increasing basicity and optimal alkyl chain length enhance use-dependent block. This was demonstrated by replacing the amino group with a more basic guanidine one while maintaining a proper distance between positive charge and aromatic ring (Me13) or with homologs having the chirality center nearby the amino group or the aromatic ring. Accordingly, a phenyl group on the asymmetric center in the homologated alkyl chain (Me12), leads to a further increase of use-dependent behavior versus the phenyl Mex derivative Me4. A fluorine atom in paraposition and one ortho-methyl group on the xylyloxy ring (Me15) increase potency and stereoselectivity versus Me4. Charge delocalization and greater flexibility of Me15 may increase its affinity for Tyr residues influencing steric drug interaction with the primary Phe residue of the binding site. Me12 and Me15 show limited selectivity against Na(v)-isoforms, possibly due to the highly conserved binding site on Na(v). To our knowledge, the new compounds are the most potent Mex-like Na(v) blockers obtained to date and deserve further investigation.

Structural modelling and mutant cycle analysis predict pharmacoresponsiveness of a Na(V)1.7 mutant channel

Sodium channel Na(V)1.7 is critical for human pain signalling. Gain-of-function mutations produce pain syndromes including inherited erythromelalgia, which is usually resistant to pharmacotherapy, but carbamazepine normalizes activation of Na(V)1.7-V400M mutant channels from a family with carbamazepine-responsive inherited erythromelalgia. Here we show that structural modelling and thermodynamic analysis predict pharmacoresponsiveness of another mutant channel (S241T) that is located 159 amino acids distant from V400M. Structural modelling reveals that Na(v)1.7-S241T is ~2.4 Å apart from V400M in the folded channel, and thermodynamic analysis demonstrates energetic coupling of V400M and S241T during activation. Atomic proximity and energetic coupling are paralleled by pharmacological coupling, as carbamazepine (30 μM) depolarizes S214T activation, as previously reported for V400M. Pharmacoresponsiveness of S241T to carbamazepine was further evident at a cellular level, where carbamazepine normalized the hyperexcitability of dorsal root ganglion neurons expressing S241T. We suggest that this approach might identify variants that confer enhanced pharmacoresponsiveness on a variety of channels.

Lidocaine reduces the transition to slow inactivation in Na(v)1.7 voltage-gated sodium channels

BACKGROUND AND PURPOSE

The primary use of local anaesthetics is to prevent or relieve pain by reversibly preventing action potential propagation through the inhibition of voltage-gated sodium channels. The tetrodotoxin-sensitive voltage-gated sodium channel subtype Na(v)1.7, abundantly expressed in pain-sensing neurons, plays a crucial role in perception and transmission of painful stimuli and in inherited chronic pain syndromes. Understanding the interaction of lidocaine with Na(v)1.7 channels could provide valuable insight into the drug's action in alleviating pain in distinct patient populations. The aim of this study was to determine how lidocaine interacts with multiple inactivated conformations of Na(v)1.7 channels.

EXPERIMENTAL APPROACH

We investigated the interactions of lidocaine with wild-type Na(v)1.7 channels and a paroxysmal extreme pain disorder mutation (I1461T) that destabilizes fast inactivation. Whole cell patch clamp recordings were used to examine the activity of channels expressed in human embryonic kidney 293 cells.

KEY RESULTS

Depolarizing pulses that increased slow inactivation of Na(v)1.7 channels also reduced lidocaine inhibition. Lidocaine enhanced recovery of Na(v)1.7 channels from prolonged depolarizing pulses by decreasing slow inactivation. A paroxysmal extreme pain disorder mutation that destabilizes fast inactivation of Na(v)1.7 channels decreased lidocaine inhibition.

CONCLUSIONS AND IMPLICATIONS

Lidocaine decreased the transition of Na(v)1.7 channels to the slow inactivated state. The fast inactivation gate (domain III-IV linker) is important for potentiating the interaction of lidocaine with the Na(v)1.7 channel.

Molecular Insights into the Local Anesthetic Receptor within Voltage-Gated Sodium Channels Using Hydroxylated Analogs of Mexiletine

We previously showed that the β-adrenoceptor modulators, clenbuterol and propranolol, directly blocked voltage-gated sodium channels, whereas salbutamol and nadolol did not (Desaphy et al., 2003), suggesting the presence of two hydroxyl groups on the aromatic moiety of the drugs as a molecular requisite for impeding sodium channel block. To verify such an hypothesis, we synthesized five new mexiletine analogs by adding one or two hydroxyl groups to the aryloxy moiety of the sodium channel blocker and tested these compounds on hNav1.4 channels expressed in HEK293 cells. Concentration–response relationships were constructed using 25-ms-long depolarizing pulses at −30 mV applied from an holding potential of −120 mV at 0.1 Hz (tonic block) and 10 Hz (use-dependent block) stimulation frequencies. The half-maximum inhibitory concentrations (IC₅₀) were linearly correlated to drug lipophilicity: the less lipophilic the drug, minor was the block. The same compounds were also tested on F1586C and Y1593C hNav1.4 channel mutants, to gain further information on the molecular interactions of mexiletine with its receptor within the sodium channel pore. In particular, replacement of Phe1586 and Tyr1593 by non-aromatic cysteine residues may help in the understanding of the role of π–π or π–cation interactions in mexiletine binding. Alteration of tonic block suggests that the aryloxy moiety of mexiletine may interact either directly or indirectly with Phe1586 in the closed sodium channel to produce low-affinity binding block, and that this interaction depends on the electrostatic potential of the drug aromatic tail. Alteration of use-dependent block suggests that addition of hydroxyl groups to the aryloxy moiety may modify high-affinity binding of the drug amine terminal to Phe1586 through cooperativity between the two pharmacophores, this effect being mainly related to drug lipophilicity. Mutation of Tyr1593 further impaired such cooperativity. In conclusion, these results confirm our former hypothesis by showing that the presence of hydroxyl groups to the aryloxy moiety of mexiletine greatly reduced sodium channel block, and provide molecular insights into the intimate interaction of local anesthetics with their receptor.

Searching for novel anti-myotonic agents: pharmacophore requirement for use-dependent block of skeletal muscle sodium channels by N-benzylated cyclic derivatives of tocainide

Drug screening on sodium currents of native myofibers by means of voltage-clamp recordings is predictive of pre-clinical anti-myotonic activity in vivo and ex vivo. By this approach we identified the N-benzylated beta-proline derivative of tocainide (To10) as the most potent use-dependent blocker of Nav1.4 so far. We tested novel analogs with modifications on the pharmacophore groups of To10. The substitution of the proline cycle with less planar piperidine or piperazine rings disclosed the importance of a two carbon atom distance and/or an additional nitrogen atom for potency. Structural changes on the xylididic group corroborated the role of a proper electronic cloud for hydrophobic interactions with the binding site. The N-benzylated moiety lead to a stereoselective behavior only in the rigid alpha-proline analog To11 vs. To10 and N-benzylated tocainide (To12). The results confirm the strict structural requirements of Nav1.4 blockers and allow to refine the drug design toward novel anti-myotonic drugs.

Role of voltage-gated sodium, potassium and calcium channels in the development of cocaine-associated cardiac arrhythmias

Cocaine is a highly active stimulant that alters dopamine metabolism in the central nervous system resulting in a feeling of euphoria that with time can lead to addictive behaviours. Cocaine has numerous deleterious effects in humans including seizures, vasoconstriction, ischaemia, increased heart rate and blood pressure, cardiac arrhythmias and sudden death. The cardiotoxic effects of cocaine are indirectly mediated by an increase in sympathomimetic stimulation to the heart and coronary vasculature and by a direct effect on the ion channels responsible for maintaining the electrical excitability of the heart. The direct and indirect effects of cocaine work in tandem to disrupt the co-ordinated electrical activity of the heart and have been associated with life-threatening cardiac arrhythmias. This review focuses on the direct effects of cocaine on cardiac ion channels, with particular focus on sodium, potassium and calcium channels, and on the contributions of these channels to cocaine-induced arrhythmias. Companion articles in this edition of the journal examine the epidemiology of cocaine use (Wood & Dargan) and the treatment of cocaine-associated arrhythmias (Hoffmann).

Analysis of the action of lidocaine on insect sodium channels

A new class of sodium channel blocker insecticides (SCBIs), which include indoxacarb, its active metabolite, DCJW, and metaflumizone, preferably block inactivated states of both insect and mammalian sodium channels in a manner similar to that by which local anesthetic (LA) drugs block mammalian sodium channels. A recent study showed that two residues in the cockroach sodium channel, F1817 and Y1824, corresponding to two key LA-interacting residues identified in mammalian sodium channels are not important for the action of SCBIs on insect sodium channels, suggesting unique interactions of SCBIs with insect sodium channels. However, the mechanism of action of LAs on insect sodium channels has not been investigated. In this study, we examined the effects of lidocaine on a cockroach sodium channel variant, BgNa(v)1-1a, and determined whether F1817 and Y1824 are also critical for the action of LAs on insect sodium channels. Lidocaine blocked BgNa(v)1-1a channels in the resting state with potency similar to that observed in mammalian sodium channels. Lidocaine also stabilized both fast-inactivated and slow-inactivated states of BgNa(v)1-1a channels, and caused a limited degree of use- and frequency-dependent block, major characteristics of LA action on mammalian sodium channels. Alanine substitutions of F1817 and Y1824 reduced the sensitivity of the BgNa(v)1-1a channel to the use-dependent block by lidocaine, but not to tonic blocking and inactivation stabilizing effects of lidocaine. Thus, similar to those on mammalian sodium channels, F1817 and Y1824 are important for the action of lidocaine on cockroach sodium channels. Our results suggest that the receptor sites for lidocaine and SCBIs are different on insect sodium channels.

Molecular determinants of state-dependent block of voltage-gated sodium channels by pilsicainide

BACKGROUND AND PURPOSE

Pilsicainide, an anti-arrhythmic drug used in Japan, is described as a pure sodium channel blocker. We examined the mechanisms by which it is able to block open channels, because these properties may be especially useful to reduce hyperexcitability in pathologies characterized by abnormal sodium channel opening.

EXPERIMENTAL APPROACH

The effects of pilsicainide on various heterologously expressed human sodium channel subtypes and mutants were investigated using the patch clamp technique.

KEY RESULTS

Pilsicainide exhibited tonic and use-dependent effects comparable to those of mexiletine and flecainide on hNav1.4 channels. These use-dependent effects were abolished in the mutations F1586C and Y1593C within segment 6 of domain IV, suggesting that the interaction of pilsicainide with these residues is critical for its local anaesthetic action. Its affinity constants for closed channels (K(R)) and channels inactivated from the closed state (K(I)) were high, suggesting that its use-dependent block (UDB) requires the channel to be open for it to reach a high-affinity blocking site. Accordingly, basic pH, which slightly increased the proportion of neutral drug, dramatically decreased K(R) and K(I) values. Effects of pilsicainide were similar on skeletal muscle hNav1.4, brain hNav1.1 and heart hNav1.5 channels. The myotonic R1448C and G1306E hNav1.4 mutants were more and less sensitive to pilsicainide, respectively, due to mutation-induced gating modifications.

CONCLUSIONS AND IMPLICATIONS

Although therapeutic concentrations of pilsicainide may have little effect on resting and closed-state inactivated channels, it induces a strong UDB due to channel opening, rendering the drug ideally suited for inhibition of high-frequency action potential firing.

Molecular model of anticonvulsant drug binding to the voltage-gated sodium channel inner pore

The tricyclic anticonvulsant drugs phenytoin, carbamazepine, and lamotrigine block neuronal voltage-gated Na(+) channels, and their binding sites to domain IV-S6 in the channel's inner pore overlap with those of local anesthetic drugs. These anticonvulsants are neutral, in contrast to the mostly positively charged local anesthetics, but their open/inactivated-state blocking affinities are similar. Using a model of the open pore of the Na(+) channel that we developed by homology with the crystal structures of potassium channels, we have docked these three anticonvulsants with residues identified by mutagenesis as important for their binding energy. The three drugs show a common pharmacophore, including an aromatic ring that has an aromatic-aromatic interaction with Tyr-1771 of Na(V)1.2 and a polar amide or imide that interacts with the aromatic ring of Phe-1764 by a low-energy amino-aromatic hydrogen bond. The second aromatic ring is nearly at a right angle to the pharmacophore and fills the pore lumen, probably interacting with the other S6 segments and physically occluding the inner pore to block Na(+) permeation. Hydrophobic interactions with this second aromatic ring may contribute an important component to binding for anticonvulsants, which compensates energetically for the absence of positive charge in their structures. Voltage dependence of block, their important therapeutic property, results from their interaction with Phe-1764, which connects them to the voltage sensors. Their use dependence is modest and this results from being neutral, with a fast drug off-rate after repolarization, allowing a normal action potential rate in the presence of the drugs.

Binding of sodium channel inhibitors to hyperpolarized and depolarized conformations of the channel

Sodium channels are inhibited by a chemically diverse group of compounds. In the last decade entirely new structural classes with superior properties have been discovered, and novel therapeutic uses of sodium channel inhibitors (SCIs) have been suggested. Many promising novel drug candidates have been described and characterized. Published structure-activity relationship studies, pharmacophore models, and mutagenesis studies seem to lag behind, dealing with only a limited group of inhibitor compounds. The abundance of novel compounds requires an organized comparison of drug potencies. The affinity of sodium channel inhibitors can vary typically ten- to thousand-fold depending on the voltage protocol; therefore comparison of electrophysiology data is difficult. In this study we describe a method for standardization of these data with the help of a simple model of state-dependence. We derived hyperpolarized (resting) and depolarized (generally termed "inactivated") state affinities for the studied drugs, which made the measurements comparable. We show a rank order of SCIs based on resting and inactivated affinity values. In an attempt to define basic chemical requirements for sodium channel inhibitor activity we investigated the dependence of both resting and inactivated state affinities on individual chemical descriptors. Lipophilicity (most often expressed by the logP value) is the single most important determinant of SCI potency. We investigated the independent impact of several other calculated chemical properties by standardizing drug potencies for logP values. By combining these two approaches: standardization of affinity values, and standardization of potencies, we concluded that while resting affinity is mostly determined by lipophilicity, inactivated state affinity is determined by a more complex interaction of chemical properties, including hydrogen bond acceptors, aromatic rings, and molecular weight.

Mechanism of action of sodium channel blocker insecticides (SCBIs) on insect sodium channels

Sodium channel blocker insecticides (SCBIs) are a relatively new class of insecticides, with a mechanism of action different from those of other classes of insecticides that target voltage-gated sodium channels. These compounds have no effect at hyperpolarized membrane potentials, but cause a voltage-dependent, nearly irreversible block as the membrane potential is depolarized. The mechanism of action of SCBIs is similar to that of local anesthetics (LAs), class I anticonvulsants and class I antiarrhythmics. In this article, we review the physiological actions of these compounds on the whole animal, the nervous system and sodium channels, and also present the results from recent studies that elucidate the receptor site of SCBIs.

Block of sensory neuronal Na+ channels by the secreolytic ambroxol is associated with an interaction with local anesthetic binding sites

Voltage-gated Na(+) channels (Na(v)) regulate the excitability of sensory neurons and are potential targets for novel analgesics. The secreolytic ambroxol reduces pain-related behavior in rodents and alleviates pain in humans. With properties resembling those of local anesthetics, ambroxol has been reported to block Na(+) currents in sensory neurons with a preference for tetrodotoxin-resistant (TTXr) Na(+) currents encoded by Na(v)1.8. However, the molecular determinants for ambroxol-induced block of Na(+) channels and a preferential block of Na(v)1.8 opposed to tetrodotoxin-sensitive (TTXs) Na(v) alpha-subunits have not been studied in detail. By means of whole-cell voltage clamp recordings, we studied the effects of ambroxol and local anesthetics on the recombinant TTXr subunit Na(v)1.8, on TTXs Na(v) alpha-subunits and on mutants of Na(v)1.4 that are insensitive to local anesthetics. Tonic and use-dependent block by ambroxol was strongly alleviated in local anesthetic-insensitive Na(v)1.4 mutants. Use-dependent block, but not tonic block was significantly stronger on Na(v)1.8 than on TTXs channels. The TTXs subunit Na(v)1.3 displayed the least degree of use-dependent block by ambroxol. The local anesthetics mepivacaine and S(-)-bupivacaine also blocked Na(v)1.8 and TTXs channels differentially. While mepivacaine displayed a preferential use-dependent block of Na(v)1.8, S(-)-bupivacaine displayed a preference for TTXs Na(+) channels. Our data show that ambroxol acts as a typical local anesthetic on Na(+) channels interacting with specific residues in the S6 segments. This property probably meditates the analgesic effect of ambroxol. Ambroxol preferentially blocks Na(v)1.8, however shares this property with established local anesthetics like mepivacaine.

Epinephrine, phenylephrine, and methoxamine induce infiltrative anesthesia via alpha1-adrenoceptors in rats

AIM

To assess whether epinephrine, phenylephrine, and methoxamine act via certain subtypes of adrenoceptors to exert their local anesthetic activity.

METHODS

We investigated cutaneous anesthesia from adrenoceptor agonists and/or antagonists in conscious, unanesthetized Sprague-Dawley male rats (weight 200-250 g). Cutaneous anesthesia was evidenced by a block of the cutaneous trunci muscle reflex, which is characterized by reflex movement of the skin over the back produced by twitches of lateral thoracispinal muscles in response to local dorsal cutaneous noxious pinprick.

RESULTS

Local infiltration of epinephrine, L-phenylephrine, or methoxamine alone induces cutaneous anesthesia in rats in a dose-dependent way. Epinephrine is found to be 19 and 29 times more potent than those of methoxamine and L-phenylephrine, respectively. The cutaneous anesthesia induced by epinephrine, phenylephrine, or methoxamine can be significantly reduced by alpha(1)-adrenoceptor antagonists (eg, prazosin), alpha1, alpha2-adrenoceptor antagonist, alpha(1A)-adrenoceptor antagonist (eg, 5-methylurapdil), alpha(1B)-adrenoceptor antagonist (eg, chloroethylclonidine), or alpha(1D)-adrenoceptor antagonist (eg, BMY7873).

CONCLUSION

Our results indicate that epinephrine, phenylephrine and methoxamine all act mainly via mixed subtypes of alpha(1)-adrenoceptors to induce cutaneous anesthesia in the rat.

Novel molecular determinants in the pore region of sodium channels regulate local anesthetic binding

The pore of the Na+ channel is lined by asymmetric loops formed by the linkers between the fifth and sixth transmembrane segments (S5-S6). We investigated the role of the N-terminal portion (SS1) of the S5-S6 linkers in channel gating and local anesthetic (LA) block using site-directed cysteine mutagenesis of the rat skeletal muscle (Na(V)1.4) channel. The mutants examined have variable effects on voltage dependence and kinetics of fast inactivation. Of the cysteine mutants immediately N-terminal to the putative DEKA selectivity filter in four domains, only Q399C in domain I and F1236C in domain III exhibit reduced use-dependent block. These two mutations also markedly accelerated the recovery from use-dependent block. Moreover, F1236C and Q399C significantly decreased the affinity of QX-314 for binding to its channel receptor by 8.5- and 3.3-fold, respectively. Oddly enough, F1236C enhanced stabilization of slow inactivation by both hastening entry into and delaying recovery from slow inactivation states. It is noteworthy that symmetric applications of QX-314 on both external and internal sides of F1236C mutant channels reduced recovery from use-dependent block, indicating an allosteric effect of external QX-314 binding on the recovery of availability of F1236C. These observations suggest that cysteine mutation in the SS1 region, particularly immediate adjacent to the DEKA ring, may lead to a structural rearrangement that alters binding of permanently charged QX-314 to its receptor. The results lend further support for a role for the selectivity filter region as a structural determinant for local anesthetic block.

Role of the sixth transmembrane segment of domain IV of the cockroach sodium channel in the action of sodium channel blocker insecticides

Sodium channel blocker insecticides (SCBIs), such as indoxacarb and metaflumizone, are a new class of insecticides with a mechanism of action different from those of other insecticides that target sodium channels. SCBIs block sodium channels in a manner similar to local anesthetics (LAs) such as lidocaine. Several residues, particularly F1579 and Y1586, in the sixth transmembrane segment (S6) of domain IV (IV) of rat Na(v)1.4 sodium channels are required for the action of LAs and SCBIs and may form part of overlapping receptor sites. However, the binding site for SCBIs in insect sodium channels remains undefined. We used site-directed mutagenesis, the Xenopus laevis oocyte expression system, and the two-electrode voltage clamp technique to study the effects on SCBI activity of mutating F1817 and Y1824 (analogous to those residues identified in mammalian sodium channels) to alanine, in the voltage-sensitive sodium channel of the German cockroach, Blattella germanica. The mutant channels showed no effect or a marked increase in channel sensitivity to both DCJW (the active metabolite of indoxacarb) and metaflumizone. Thus, it appeared that although the F1817 residue plays a role in the action of SCBIs and that both residues are involved in LA activity in mammalian sodium channels, neither F1817 nor Y1824 are integral determinants of SCBI binding on insect sodium channels. Our results suggest that the receptor site of SCBIs on insect sodium channels may be significantly different from that on mammalian sodium channels.

Structural determinants of drugs acting on the Nav1.8 channel

The aim of this work is to study the role of pore residues on drug binding in the Na(V)1.8 channel. Alanine mutations were made in the S6 segments, chosen on the basis of their roles in other Na(V) subtypes; whole cell patch clamp recordings were made from mammalian ND7/23 cells. Mutations of some residues caused shifts in voltage dependence of activation and inactivation, and gave faster time course of inactivation, indicating that the residues mutated play important roles in both activation and inactivation in the Na(V)1.8 channel. The resting and inactivated state affinities of tetracaine for the channel were reduced by mutations I381A, F1710A, and Y1717A (for the latter only inactivated state affinity was measured), and by mutation F1710A for the Na(V)1.8-selective compound A-803467, showing the involvement of these residues for each compound, respectively. For both compounds, mutation L1410A caused the unexpected appearance of a complete resting block even at extremely low concentrations. Resting block of native channels by compound A-803467 could be partially removed ("disinhibition") by repetitive stimulation or by a test pulse after recovery from inactivation; the magnitude of the latter effect was increased for all the mutants studied. Tetracaine did not show this effect for native channels, but disinhibition was seen particularly for mutants L1410A and F1710A. The data suggest differing, but partially overlapping, areas of binding of A-803467 and tetracaine. Docking of the ligands into a three-dimensional model of the Na(V)1.8 channel gave interesting insight as to how the ligands may interact with pore residues.

Outward stabilization of the S4 segments in domains III and IV enhances lidocaine block of sodium channels

The anti-arrhythmic drug lidocaine has been shown to have a lower affinity for block of voltage-gated sodium channels at hyperpolarized potentials compared to depolarized potentials. Concomitantly, lidocaine reduces maximum gating charge (Qmax) by 40% resulting from the complete stabilization of the S4 in domain III in an outward, depolarized position and partial stabilization of the S4 in domain IV in wild-type Na+ channels (Na(V)1.5). To investigate whether the pre-positioning of the S4 segments in these two domains in a depolarized conformation increases affinity for lidocaine block, a cysteine residue was substituted for the 3rd outermost charged residue in the S4 of domain III (R3C-DIII) and for the 2nd outermost Arg in S4 of domain IV (R2C-DIV) in Na(V)1.5. After biotinylation by exposure to extracellular MTSEA-biotin the mutated S4s became stabilized in an outward, depolarized position. For Na+ channels containing both mutations (R3C-DIII + R2C-DIV) the IC50 for rested-state lidocaine block decreased from 194 +/- 15 microM in control to 28 +/- 2 microM after MTSEA-biotin modification. To determine whether an intact inactivation gate (formed by the linker between domains III and IV) was required for local anaesthetic drugs to modify Na+ channel gating currents, a Cys was substituted for the Phe in the IFM motif of the inactivation gate (ICM) and then modified by intracellular MTSET (WT-ICM(MTSET)) before exposure to intracellular QX-222, a quarternary amine. Although WT-ICM(MTSET) required higher concentrations of drug to block I(Na) compared to WT, Qmax decreased by 35% and the V1/2 shifted leftward as previously demonstrated for WT. The effect of stabilization of the S4s in domains III and IV in the absence of an intact inactivation gate on lidocaine block was determined for R3C-DIII + ICM, R2C-DIV + ICM and R3C-DIII + R2C-DIV + ICM, and compared to WT-ICM. IC50 values were 1360 +/- 430 microM, 890 +/- 70 microM, 670 +/- 30 microM and 1920 +/- 60 microM, respectively. Thermodynamic mutant-cycle analysis was consistent with additive (i.e. independent) contributions from stabilization of the individual S4s in R3C-DIII + ICM and R2C-DIV + ICM. We conclude that the positions of the S4s in domains III and IV are major determinants of the voltage dependence of lidocaine affinity.

A Nav1.7 channel mutation associated with hereditary erythromelalgia contributes to neuronal hyperexcitability and displays reduced lidocaine sensitivity

Mutations in the TTX-sensitive voltage-gated sodium channel subtype Nav1.7 have been implicated in the painful inherited neuropathy, hereditary erythromelalgia. Hereditary erythromelalgia can be difficult to treat and, although sodium channels are targeted by local anaesthetics such as lidocaine (lignocaine), some patients do not respond to treatment with local anaesthetics. This study examined electrophysiological differences in Nav1.7 caused by a hereditary erythromelalgia mutation (N395K) that lies within the local anaesthetic binding site of the channel. The N395K mutation produced a hyperpolarized voltage dependence of activation, slower kinetics of deactivation, and impaired steady-state slow inactivation. Computer simulations indicate that the shift in activation is the major determinant of the hyperexcitability induced by erythromelalgia mutations in sensory neurons, but that changes in slow inactivation can modulate the overall impact on excitability. This study also investigated lidocaine inhibition of the Nav1.7-N395K channel. We show that the N395K mutation attenuates the inhibitory effects of lidocaine on both resting and inactivated Nav1.7. The IC50 for lidocaine was estimated at 500 microM for inactivated wild-type Nav1.7 and 2.8 mM for inactivated Nav1.7-N395K. The N395K mutation also significantly reduced use-dependent inhibition of lidocaine on Nav1.7 current. In contrast, a different hereditary erythromelalgia mutation (F216S), not located in the local anaesthetic binding site, had no effect on lidocaine inhibition of Nav1.7 current. Our observation of reduced lidocaine inhibition on Nav1.7-N395K shows that the residue N395 is critical for lidocaine binding to Nav1.7 and suggests that the response of individuals with hereditary erythromelalgia to lidocaine treatment may be determined, at least in part, by their specific genotype.

Local anesthetics

Local anesthetics are used broadly to prevent or reverse acute pain and treat symptoms of chronic pain. This chapter, on the analgesic aspects of local anesthetics, reviews their broad actions that affect many different molecular targets and disrupt their functions in pain processing. Application of local anesthetics to peripheral nerve primarily results in the blockade of propagating action potentials, through their inhibition of voltage-gated sodium channels. Such inhibition results from drug binding at a site in the channel's inner pore, accessible from the cytoplasmic opening. Binding of drug molecules to these channels depends on their conformation, with the drugs generally having a higher affinity for the open and inactivated channel states that are induced by membrane depolarization. As a result, the effective potency of these drugs for blocking impulses increases during high-frequency repetitive firing and also under slow depolarization, such as occurs at a region of nerve injury, which is often the locus for generation of abnormal, pain-related ectopic impulses. At distal and central terminals the inhibition of voltage-gated calcium channels by local anesthetics will suppress neurogenic inflammation and the release of neurotransmitters. Actions on receptors that contribute to nociceptive transduction, such as TRPV1 and the bradykinin B2 receptor, provide an independent mode of analgesia. In the spinal cord, where local anesthetics are present during epidural or intrathecal anesthesia, inhibition of inotropic receptors, such as those for glutamate, by local anesthetics further interferes with neuronal transmission. Activation of spinal cord mitogen-activated protein (MAP) kinases, which are essential for the hyperalgesia following injury or incision and occur in both neurons and glia, is inhibited by spinal local anesthetics. Many G protein-coupled receptors are susceptible to local anesthetics, with particular sensitivity of those coupled via the Gq alpha-subunit. Local anesthetics are also infused intravenously to yield plasma concentrations far below those that block normal action potentials, yet that are frequently effective at reversing neuropathic pain. Thus, local anesthetics modify a variety of neuronal membrane channels and receptors, leading to what is probably a synergistic mixture of analgesic mechanisms to achieve effective clinical analgesia.

A common SCN5A variant alters the responsiveness of human sodium channels to class I antiarrhythmic agents

BACKGROUND

The potential pathophysiological role of common SCN5A polymorphisms in cardiac arrhythmias has been increasingly recognized. However, little is known about the impact of those polymorphisms on the pharmocological response of hNav1.5 to various antiarrhythmic agents.

METHODS AND RESULTS

The known SCN5A polymorphism, S524Y, was studied in comparison with the wild type (WT) in [corrected] the SCN5A-Q1077del variant. The ion channel gating kinetics and pharmacology were evaluated using whole-cell patch-clamp methods in HEK-293 cells. Consistent with a previous report, the basal ion channel gating kinetics of S524Y were indistinguishable from the WT. Quinidine (20 microM) caused similar extent of tonic block reduction of sodium currents at -120 mV in WT and S524Y. Surprisingly, quinidine (20 microM) exerted a more use-dependent block by a 10 Hz pulse train in S524Y than in WT at 22 degrees C (Ki: WT, 51.3 microM; S524Y, 20.3 microM). S524Y significantly delayed recovery from the use-dependent block, compared with the WT (tau= 88.6 +/- 7.9 s vs 41.9 +/- 6.6 s, P < 0.005). Under more physiological conditions using a 2 Hz pulse train at 37 degrees C, S524Y similarly enhanced the use-dependent block by quinidine. In addition, S524Y enhanced the use-dependent block by flecainide (12.5 microM), but not by mexiletine (100 microM).

CONCLUSION

A common SCN5A polymorphism, S524Y, can enhance a use-dependent block by class Ia and Ic antiarrhythmic agents. Our findings may have clinical implications in pharmacological management of cardiac arrhythmias since this common SCN5A polymorphism might be a contributing factor to the variable antiarrhythmic response.

Serine-401 as a batrachotoxin- and local anesthetic-sensing residue in the human cardiac Na+ channel

Sequence alignment of four S6 segments in the human cardiac Na+ channel suggests that serine-401 (hNav1.5-S401) at D1S6 along with asparagine-927 (N927) at D2S6, serine-1458 (S1458) at D3S6, and phenylalanine-1760 (F1760) at D4S6 may jointly form a pore-facing S(401)N(927)S(1458)F(1760) ring. Importantly, this pore-facing structure is adjacent to the putative gating-hinge (G(400)G(926)G(1457)S(1759)) and close to the selectivity filter. Within this SNSF ring, only S401 has not yet been identified as a batrachotoxin (BTX) sensing residue. We therefore created S401 mutants with 12 substitutions (S401C,W,P,A,K,F,R,E,L,N,D,G) and assayed their BTX sensitivity. All S401 mutants expressed Na+ currents but often with altered gating characteristics. Ten mutants were found sensitive to 5 muM BTX, which eliminated Na+ channel fast inactivation after repetitive pulses. However, S401K and S401R became BTX resistant. In addition, the block of open and inactivated hNav1.5-S401K Na+ channels by local anesthetic bupivacaine was reduced by approximately 8-10-fold, but not the block of resting Na+ channels. Qualitatively, these ligand-sensing phenotypes of hNav1.5-S401K channels resemble those of S1458K and F1760K channels reported earlier. Together, our results support that residue hNav1.5-S401 at D1S6 is facing the inner cavity and is in close proximity to the receptor sites for BTX and for local anesthetics.

Preemptive analgesia with lidocaine prevents Failed Back Surgery Syndrome

Failed Back Surgery Syndrome (FBSS) is commonly encountered in pain-treatment settings in the United States. We tested whether potential key factors in this syndrome, such as extracellular concentrations of excitatory amino acids (EAAs), are increased in the dorsal horn by synaptic release due to unintentional stretch and/or deformation/compression/transection of dorsal spinal structures during surgery. We hypothesized that pharmacological nerve block as a form of preemptive analgesia prior to any insult to dorsal root neurons will prevent an abnormally high increase in extracellular concentrations of EAAs in the dorsal horn and ultimately the establishment of central sensitization during back surgery. The L4 and L5 dorsal roots were cut bilaterally near the spinal cord to provide an adequate model to test for preemptive analgesia. Amino acid concentrations were measured by dorsal horn microdialysis sampling; EAAs aspartate and glutamate were significantly increased by 80% and 65% respectively, as were other amino acids compared to sham control values. Topical application of 1% Lidocaine, a voltage-gated Na(+) channel blocker, for 10 min prior to L4 and L5 bilateral dorsal rhizotomy (BDR) significantly attenuated the increase in EAA concentrations such that their values were not different from sham controls. Behavioral tests demonstrated significant hindlimb mechanical allodynia after BDRs that was significantly attenuated by Lidocaine pretreatment. Thus, Lidocaine pretreatment could offer a safe measure for prevention of chronic pain for back surgical procedures if given by intramuscular injection, topical administration onto spinal nerves and/or the dorsal spinal surface during surgical procedures that include nerve entrapment release, intervertebral disc modification and laminectomies.

Use-Dependent Block by Lidocaine but Not Amitriptyline Is More Pronounced in Tetrodotoxin (TTX)-Resistant Nav1.8 Than in TTX-Sensitive Na+ Channels

The excitability of sensory neurons depends on the expression of various voltage-gated Na+ channel isoforms. The tetrodotoxin-resistant (TTXr) Na+ channel Na(v)1.8 accounts for the electroresponsiveness of nociceptive neurons and contributes to inflammatory and neuropathic pain. Na+ channel blockers are clinically employed for chronic pain management, but side effects limit their use. There is conflicting information whether their potency to block tetrodotoxin-sensitive (TTXs) and TTXr Na+ channels differs. We analyzed the action of lidocaine and amitriptyline on TTXr Na(v)1.8 heterologously expressed in ND7/23 cells in comparison with TTXs Na+ channels endogenously expressed in ND7/23 cells. TTXr Na(v)1.8 and TTXs currents were investigated under whole-cell voltage-clamp. At a holding potential of -80 mV, lidocaine was 5-fold and amitriptyline 8-fold more potent to tonically block TTXs than Na(v)1.8 currents. This was due to a higher percentage of TTXs channels residing in the inactivated, high-affinity state at this potential. Tonic block of either resting or inactivated channels by lidocaine or amitriptyline revealed little differences between TTXs and Na(v)1.8 channels. Use-dependent block by amitriptyline was similar in TTXs and Na(v)1.8 channels. Surprisingly, use-dependent block by lidocaine was more pronounced in Na(v)1.8 than in TTXs channels. This result was confirmed in dorsal root ganglion neurons and is associated with the greater tendency of Na(v)1.8 to enter a slow inactivated state. Our data suggest that lidocaine could selectively block Na(v)1.8-mediated action potential firing. It is conceivable that the expression pattern of Na+ channels in sensory neurons might influence the efficiency of Na+ channel blockers used for chronic pain management.

Evaluation of the pharmacological activity of the major mexiletine metabolites on skeletal muscle sodium currents

BACKGROUND AND PURPOSE

Mexiletine (Mex), an orally effective antiarrhythmic agent used to treat ventricular arrhythmias, has also been found to be effective for myotonia and neuropathic pain. It is extensively metabolized in humans but little information exists about the pharmacodynamic properties of its metabolites.

EXPERIMENTAL APPROACH

To determine their contribution to the clinical activity of Mex, p-hydroxy-mexiletine (PHM), hydroxy-methyl-mexiletine (HMM), N-hydroxy-mexiletine (NHM) (phase I reaction products) and N-carbonyloxy beta-D-glucuronide (NMG) (phase II reaction product) were tested on sodium currents (I(Na)) of frog skeletal muscle fibres. Sodium currents were elicited with depolarizing pulses from different holding potentials (HP=-140, -100, -70 mV) and stimulation frequencies (0.25, 0.5, 1, 2, 5, 10 Hz) using the vaseline-gap voltage-clamp method.

KEY RESULTS

All the hydroxylated derivatives blocked the sodium channel in a voltage- and use-dependent manner. The PHM, HMM and NHM metabolites were up to 10-fold less effective than the parent compound. However, HMM showed a greater use-dependent behaviour (10 Hz), compared to Mex and the other metabolites. Similar to Mex, these products behaved as inactivating channel blockers. Conjugation with glucuronic acid (NMG) resulted in almost complete abolition of the pharmacological activity of the parent compound.

CONCLUSIONS AND IMPLICATIONS

Thus, although less potent, the phase I metabolites tested demonstrated similar pharmacological behaviour to Mex and might contribute to its clinical profile.

An inner pore residue (Asn406) in the Nav1.5 channel controls slow inactivation and enhances mibefradil block to T-type Ca2+ channel levels

Mibefradil is a tetralol derivative once marketed to treat hyper-tension. Its primary target is the T-type Ca(2+) channel (IC(50), approximately 0.1-0.2 microM), but it also blocks Na(+),K(+),Cl(-), and other Ca(2+) channels at higher concentrations. We have recently reported state-dependent mibefradil block of Na(+) channels in which apparent affinity was enhanced when channels were recruited to slow-inactivated conformations. The structural determinants controlling mibefradil block have not been identified, although evidence suggests involvement of regions near or within the inner pore. We tested whether mibefradil interacts with the local anesthetic (LA) binding site, which includes residues in the S6 segments of domains (D) I, III, and IV. Mutagenesis of DIII S6 and DIVS6 did not reveal critical binding determinants. Substitution of Asn406 in DI S6 of cardiac Na(v)1.5, however, altered affinity in a manner dependent on the identity of the substituting residue. Replacing Asn406 with a phenylalanine or a cysteine increased affinity by 4- and 7-fold, respectively, thus conferring T-type Ca(2+) channel-like mibefradil sensitivity to the Na(+) channel. A series of other substitutions that varied in size, charge, and hydrophobicity had minimal effects on mibefradil block, but all mutations dramatically altered the magnitude and voltage-dependence of slow inactivation, consistent with data in other isoforms. Channels did not slow-inactivate, however, at the voltages used to assay mibefradil block, supporting the idea that Asn406 lies within or near the mibefradil binding site.

Interaction of local anesthetics with a peptide encompassing the IV/S4–S5 linker of the Na+ channel

The peptide pIV/S4-S5 encompasses the cytoplasmic linker between helices S4-S5 in domain IV of the voltage-gated Na+ channel, residues 1644-1664. The interaction of two local anesthetics (LA), lidocaine and benzocaine, with pIV/S4-S5 has been studied by DOSY, heteronuclear NMR 1H-15N-HSQC spectroscopy and computational methods. DOSY indicates that benzocaine, a neutral ester, exhibits stronger interaction with pIV/S4-S5 than lidocaine, a charged amine-amide. Weighted average chemical shifts, Deltadelta(1H-15N), show that benzocaine affects residues L1653, M1655 and S1656 while lidocaine slightly perturbs residues I1646, L1649 and A1659, L1660, near the N- and C-terminus, respectively. Computational methods confirmed the stability of the benzocaine binding and the existence of two binding sites for lidocaine. Even considering that the approach of studying the peptide in the presence of a co-solvent (TFE/H2O, 30%/70% v/v) has an inherently limited implication, our data strongly support the existence of multiple LA binding sites in the IV/S4-S5 linker, as suggested in the literature. In addition, we consider that LA can bind to the S4-S5 linker with diverse binding modes and strength since this linker is part of the receptor for the "inactivation gate particle". Conditions for devising new functional studies, aiming to better understand Na+ channel functionality as well as the various facets of LA pharmacological activity are proposed in this work.

Inhibition of Nav1.7 and Nav1.4 sodium channels by trifluoperazine involves the local anesthetic receptor

The calmodulin (CaM) inhibitor trifluoperazine (TFP) can produce analgesia when given intrathecally to rats; however, the mechanism is not known. We asked whether TFP could modulate the Na(v)1.7 sodium channel, which is highly expressed in the peripheral nervous system and plays an important role in nociception. We show that 500 nM and 2 muM TFP induce major decreases in Na(v)1.7 and Na(v)1.4 current amplitudes and that 2 muM TFP causes hyperpolarizing shifts in the steady-state inactivation of Na(v)1.7 and Na(v)1.4. CaM can bind to the C-termini of voltage-gated sodium channels and modulate their functional properties; therefore we investigated if TFP modulation of sodium channels was due to CaM inhibition. However, the TFP inhibition was not replicated by whole cell dialysis of a calmodulin inhibitory peptide, indicating that major effects of TFP do not involve a disruption of CaM-channel interactions. Rather, our data show that TFP inhibition is state dependent and that the majority of the TFP inhibition depends on specific amino-acid residues in the local anesthetic receptor site in sodium channels. TFP was also effective in vivo in causing motor and sensory blockade after subfascial injection to the rat sciatic nerve. The state-dependent block of Na(v)1.7 channels with nanomolar concentrations of TFP raises the possibility that TFP, or TFP analogues, might be useful for regional anesthesia and pain management and could be more potent than traditional local anesthetics.

Neuromodulation of Na+ channel slow inactivation via cAMP-dependent protein kinase and protein kinase C

Neurotransmitters modulate sodium channel availability through activation of G protein-coupled receptors, cAMP-dependent protein kinase (PKA), and protein kinase C (PKC). Voltage-dependent slow inactivation also controls sodium channel availability, synaptic integration, and neuronal firing. Here we show by analysis of sodium channel mutants that neuromodulation via PKA and PKC enhances intrinsic slow inactivation of sodium channels, making them unavailable for activation. Mutations in the S6 segment in domain III (N1466A,D) either enhance or block slow inactivation, implicating S6 segments in the molecular pathway for slow inactivation. Modulation of N1466A channels by PKC or PKA is increased, whereas modulation of N1466D is nearly completely blocked. These results demonstrate that neuromodulation by PKA and PKC is caused by their enhancement of intrinsic slow inactivation gating. Modulation of slow inactivation by neurotransmitters acting through G protein-coupled receptors, PKA, and PKC is a flexible mechanism of cellular plasticity controlling the firing behavior of central neurons.

Modeling the pore structure of voltage-gated sodium channels in closed, open, and fast-inactivated conformation reveals details of site 1 toxin and local anesthetic binding

In this work molecular modeling was applied to generate homology models of the pore region of the Na(v)1.2 and Na(v)1.8 isoforms of human voltage-gated sodium channels. The models represent the channels in the resting, open, and fast-inactivated states. The transmembrane portions of the channels were based on the equivalent domains of the closed and open conformation potassium channels KcsA and MthK, respectively. The critical selectivity loops were modeled using a structural template identified by a novel 3D-search technique and subsequently merged with the transmembrane portions. The resulting draft models were used to study the differences of tetrodotoxin binding to the tetrodotoxin-sensitive Na(v)1.2 (EC50: 0.012 microM) and -insensitive Na(v)1.8 (EC50: 60 microM) isoforms, respectively. Furthermore, we investigated binding of the local anesthetic tetracaine to Na(v)1.8 (EC50: 12.5 microM) in resting, conducting, and fast-inactivated state. In accordance with experimental mutagenesis studies, computational docking of tetrodotoxin and tetracaine provided (1) a description of site 1 toxin and local anesthetic binding sites in voltage-gated sodium channels. (2) A rationale for site 1 toxin-sensitivity versus -insensitivity in atomic detail involving interactions of the Na(v)1.2 residues F385-I and W943-II. (3) A working hypothesis of interactions between Na(v)1.8 in different conformational states and the local anesthetic tetracaine.

How batrachotoxin modifies the sodium channel permeation pathway: computer modeling and site-directed mutagenesis

A structural model of the rNav1.4 Na+ channel with batrachotoxin (BTX) bound within the inner cavity suggested that the BTX pyrrole moiety is located between a lysine residue at the DEKA selectivity filter (Lys1237) and an adjacent phenylalanine residue (Phe1236). We tested this pyrrole-binding model by site-directed mutagenesis of Phe1236 at D3/P-loop with 11 amino acids. Mutants F1236D and F1236E expressed poorly, whereas nine other mutants either expressed robust Na+ currents, like the wild-type (F1236Y/Q/K), or somewhat reduced current (F1236G/A/C/N/W/R). Gating properties were altered modestly in most mutant channels, with F1236G displaying the greatest shift in activation and steady-state fast inactivation (-10.1 and -7.5 mV, respectively). Mutants F1236K and F1236R were severely resistant to BTX after 1000 repetitive pulses (+50 mV/20 ms at 2 Hz), whereas seven other mutants were sensitive but with reduced magnitudes compared with the wild type. It is noteworthy that rNav1.4-F1236K mutant Na+ channels remained highly sensitive to block by the local anesthetic bupivacaine, unlike several other BTX-resistant mutant channels. Our data thus support a model in which BTX, when bound within the inner cavity, interacts with the D3/P-loop directly. Such a direct interaction provides clues on how BTX alters the Na+ channel selectivity and conductance.

Evidence for a multivalent interaction of symmetrical, N-linked, lidocaine dimers with voltage-gated Na+ channels

The interaction of symmetrical lidocaine dimers with voltage-gated Na+ channels (VGSCs) was examined using a FLIPR membrane potential assay and voltage-clamp. The dimers, in which the tertiary amines of the lidocaine moieties are linked by an alkylene chain (two to six methylene units), inhibited VGSC activator-evoked depolarization of cells heterologously-expressing rat (r) Na(v)1.2a, human (h) Na(v)1.5, and rNa(v)1.8, with potencies 10- to 100-fold higher than lidocaine (compound 1). The rank order of potency (C4 (compound 4) > C3 (compound 3) > or = C2 (compound 2) = C5 (compound 5) = C6 (compound 6) >> compound 1) was similar at each VGSC. Compound 4 exhibited strong use-dependent inhibition of hNa(v)1.5 with pIC50 values < 4.5 and 6.0 for tonic and phasic block, respectively. Coincubation with local anesthetics but not tetrodotoxin attenuated compound 4-mediated inhibition of hNa(v)1.5. These data suggest that the compound 4 binding site(s) is identical, or allosterically coupled, to the local anesthetic receptor. The dissociation rate of the dimers from hNa(v)1.5 was dependent upon the linker length, with a rank order of compound 1 > compound 5 = compound 6 > compound 2 >> compound 3. The observation that both the potency and dissociation rate of the dimers was dependent upon linker length is consistent with a multivalent interaction at VGSCs. hNa(v)1.5 VGSCs did not recover from inhibition by compound 4. However, "chase" with free local anesthetic site inhibitors increased the rate of dissociation of compound 4. Together, these data support the hypothesis that compound 4 simultaneously occupies two binding sites on VGSCs, both of which can be bound by known local anesthetic site inhibitors.

Sodium Channel Inactivation: Molecular Determinants and Modulation

Voltage-gated sodium channels open (activate) when the membrane is depolarized and close on repolarization (deactivate) but also on continuing depolarization by a process termed inactivation, which leaves the channel refractory, i.e., unable to open again for a period of time. In the “classical” fast inactivation, this time is of the millisecond range, but it can last much longer (up to seconds) in a different slow type of inactivation. These two types of inactivation have different mechanisms located in different parts of the channel molecule: the fast inactivation at the cytoplasmic pore opening which can be closed by a hinged lid, the slow inactivation in other parts involving conformational changes of the pore. Fast inactivation is highly vulnerable and affected by many chemical agents, toxins, and proteolytic enzymes but also by the presence of β-subunits of the channel molecule. Systematic studies of these modulating factors and of the effects of point mutations (experimental and in hereditary diseases) in the channel molecule have yielded a fairly consistent picture of the molecular background of fast inactivation, which for the slow inactivation is still lacking.

Molecular modeling of local anesthetic drug binding by voltage-gated sodium channels

Voltage-gated sodium (Na+) channels are targets for local anesthetic (LA) drugs that bind in the inner pore of the channel with affinities related to the channel gating states. Our core model of the sodium channel (P loops and S5 and S6 segments from each of the four domains) was closed because it was developed using coordinates from the KcsA channel crystallographic structure. We developed a model of the activated, open channel based on the structure of the open MthK channel, which was characterized by bends at the S6 glycine or serine residues. This created a conformation that allowed energetically appropriate docking of the LA drugs. The alkylamino head of ionizable LA molecules was docked closer to the selectivity filter and in association with Phe-1579 of IVS6 and Leu-1280 of IIIS6 (Nav1.4), and the aromatic ring interacted with Tyr-1586 of IVS6 and Asn-434 of IS6. Comparison of multiple LA drugs showed relative binding affinities in the model consistent with experimental studies. The ionizable LA alkylamino heads interact primarily by van der Waals forces that position the charge so as to create a positive electrostatic barrier for cation permeation. Permanently uncharged benzocaine could be docked in the closed conformation as well, stabilizing the closed conformation. The structurally different anticonvulsant lamotrigine and one of its derivatives have a binding site that fully overlaps with that of the LA drugs. The open, activated channel creates the high-affinity binding site for these sodium channel blocker drugs, and block may be mainly electrostatic.